Metal-oxide-semiconductor (MOS) transistors are of the basic devices in semiconductor manufacturing and are widely used in integrated circuits (ICs). Depending on the doping type, MOS transistors include NMOS and PMOS transistors.
FIGS. 1-3 show cross-sectional structures of a conventional transistor during its formation. As shown in FIG. 1, a well region 101 is formed in a semiconductor substrate 100 by ion implantation. A gate structure is formed on surface of the semiconductor substrate 100. The gate structure includes a gate electrode 103 on a gate oxide layer 102. The gate oxide layer 102 is disposed on the semiconductor substrate 100.
As shown in FIG. 2, a slightly doped region 104 is formed within the semiconductor substrate 100 on both sides of the gate structure by an ion implantation.
As shown in FIG. 3, a sidewall spacer 105 is formed on sidewalls of the gate structure. The gate structure and the sidewall spacer 105 are used as a mask to heavily dope regions in the semiconductor substrate 100 on both sides of the gate structure to form source/drain regions 106.
However, transistors fabricated by conventional methods have low carrier mobility in the channel region, which affects device performance. It is therefore desirable to provide transistors with improved performance and methods for forming such transistors.